Support language switcher and multilingual

jjchinquist → credited bbrala → .

Boris will organise how to move on with this issue.

User story for Multi lingual to be discussed and prioritized at DrupalCamp Berlin:
https://docs.google.com/document/d/19eZVGL1BYc-W5kAWR9SMfQsb92RJyjP6FxhG...

User Story Multi Language
As a visitor of drupal.xx, I can switch the language of the website with one button
As a visitor, I can choose the language of the website with a button in the topmenu
As a visitor with a visual or physical impairment, I can easily use the language switch
As a visitor, I can easily use the language switch with my reader

As an editor, I can add translations for menu items in different languages
As an editor, I can add the same content in different languages so news items, pages, forms, events and cases can easily be shown in all available languages

As a member, I can easily add translations for the events that I create
As a member, I can easily add translations for the texts on my profile
As a member, I get the automatic announcement and communication through the website in the language I prefer

As an administrator, the system supports a multilingual instance.
As an administrator, I can add / edit / remove languages from the multilingual instance. This is documented.

Part of this is tackled in 🐛 Layout Builder translation issues Active - to fix the layout_builder_at issues currently in the project.